What Causes Severe Pain At The Back Of Knees?

Hello, I work as a chef and I stand for the whole shift (9hrs) without seating. My knees getting tired very quickly and feel small pain at the back of my left knee. The tiredness comes after one hour of working. The pain restrict me of getting on my knees down and hurts when want to retract the leg. I have to sit for 30 minutes to rest and then my knees and the pain at back of my left knee resolves. I understand that working long hours brings the tiredness in my knees, but the pain at back of my left knee (ligaments/tendons or something else) worries me. Can not retract fully my left leg. Do you have any answers for that? Thank you in advance